In addition to the previously described functions, an active filter may be used to perform a band-reject function. A filter with a band-reject characteristic is frequently referred to as a notch filter. A typical circuit using a uA741 in unity-gain configuration for this type of active filter is shown in Figure 2-30.
The filter response curve shown in Figure 2-31 is a second-order band-reject filter with a notch frequency of
3 kHz. The resulting Q ofthis filter is about 23, with a notch depth of – 31 dB. Although three passive T networks are used in this application, the operational amplifier has become a sharply tuned low-frequency filter without the use of inductors or large-value capacitors.